AOT) PKTSIOLOaY OF THE SPONGIADJE. 
769 
Prehensile Spicula. 
I have so fully described, in the first part of this paper, the prehensile spicula found at 
the base of the beautiful Euplectella^ in the possession of Dr. Arthue Farre, and figured 
in Plate XXIIL fig. 53, Phil. Trans, for 1858, as to render it necessary to say but very little 
more regarding them. Fig. 44, Plate XXVI. of the same paper presents so many points 
of structural agreement with that from Eu^lectella, as to induce a very strong suspicion 
of its having had a similar office to perform in the sponge from which it was obtained ; 
but its extremely small size is against that supposition, and in favour of its being an in- 
ternal defensive one. Both sponges producing these forms were parasitical on other 
sponges. With respect to the larger form there is no reasonable doubt of its office in the 
sponge, and the smaller ones may have been basal appendages to a very small species. 
I have searched other species of Euplectella in vain for similar forms. 
Defensive Spicula. 
The modes of defence in the Spongiadce by means of spicula are exceedingly various, 
and in many cases remarkably complex and interesting. They may be divided into two 
great systems, — 1st, those of external defence ; and 2nd, those of internal defence. 
If I were to attempt to enter upon a description of every variation in the mode of the 
application of spicula to defensive purposes, it would extend this portion of the subject 
to a greater length than we can afford under the present circumstances. I shall therefore 
confine my observations to a description of the general principles of defence as exhibited 
in some of the principal genera of the Spongia,dee. 
In the external defences, the mode of the application of the spicula depends in a great 
degree on the structure of the skeleton of the sponge. The most simple cases are those 
where the structure of the skeleton consists of spicula radiating from the centre or the 
axes of the sponge, and in these cases they usually consist of the terminations of the radial 
lines of the skeleton, the distal spicula of which are frequently projected for a considerable 
part of their length through the dermal membranes, and in many sponges the surface is 
thus thickly studded with them ; and in species where the terminal radial lines of the 
skeleton contain many spicula, they are frequently found at their apices to assume a 
radiating direction, so as to present the greatest possible number of points to their external 
enemies. This mode of defence is very general in the numerous British species of the 
genera Isodictija and Chalina, Bowerbaxk, MS. Fig. 10, Plate XXIX. represents a 
small portion of a section at right angles to the surface from Halichonclria seriata, 
JoHNSTOX, Chalina., Botverbank, MS., illustrating very distinctly this simple mode of 
external defence. 
In the genus Dictyocylindrus., Bowerbaxk, MS., which consists principally of slender 
branching sponges, many of which in their living state are exceedingly fleshy in their 
appearance, the skeleton is formed of a central cylinder, composed of a network of 
spicula, from the surface of which radiate in vast quantities long, slender and acutely 
]3ointed spicula, which in the living condition project slightly beyond the dermal mem- 
